This invention relates generally to surface acoustic wave (SAW) devices and, more particularly, to surface acoustic wave devices having low vibration sensitivity.
As it is known in the art, surface acoustic wave (SAW) devices are employed in a variety of applications, such as resonators and delay lines for oscillator circuits, as well as, filters and pressure transducers. Generally, a SAW device comprises at least one transducer including a set of conductive members which is disposed on or recessed within a surface of a piezoelectric substrate.
In many applications of SAW devices, particularly with respect to applications of resonators and delay lines, as frequency stabilizing and determining elements in an oscillator circuit, it is important to provide a housing having a relatively small size in order to reduce the so-called vibration sensitivity of the SAW device. At the same time, the housing must be large enough to accommodate the SAW device along with additional, associated electronic circuitry used to complete the oscillator circuit.
It is known that the resonant frequency of an oscillator circuit, including a SAW device, is sensitive of external vibration or changes in external stress applied to the SAW device. The effect of mechanical vibration may degrade the performance of a SAW device in an oscillator circuit application, for example, by modulating the output frequency of the oscillator circuit. The frequency modulation occurs due to deflections of the SAW device substrate through which surface waves propagate. These deflections can be generated internally due to vibrations in the mass of the substrate, or can be externally induced from vibratory bending of the housing. Deflections due solely to vibrations of the mass of the SAW device substrate have been discussed in the following articles: "An Analysis of the Normal Acceleration Sensitivity of ST-Cut Quartz Surface wave Resonators Rigidly Supported Along the Edges" by H. F. Tiersten and D. V. Schick, Proc. of the 41st Ann. Symp. on Freq. Control., 1987, pp. 282; and "Improved Vibration Sensitivity of the All Quartz Package Surface Acoustic Wave Resonator", by J. A. Greer and T. E. Parker, Proc. of the 42nd Ann. Symp. on Freq. Control, 1988, pp. 239. The sensitivity results from the applied acceleration on the piezoelectric substrate causing changes in surface wave velocity and hence resonant frequency characteristics of the SAW device. In particular, the surface wave velocity is influenced by applied forces through the second order elastic coefficients of the material of the SAW substrate and also because the physical distance between the transducers or gratings on the substrate is changed. Thus, both of these factors contribute to net frequency change in SAW devices such as delay lines and resonators.
With regard to externally induced vibrations caused from bending of the housing, one approach has been to attach externally mounted stiffeners to the housing to reduce the amount of vibratory bending of the housing. Such technique is described in U.S. Pat. No. 5,208,504, entitled "SAW Device and Method of Manufacture", inventors, Thomas E. Parker and James A. Greer, issued May 4, 1993 and assigned to the same assignee as the present invention. The patent describes the use of a housing containing the SAW device and its associated electronic circuitry. Mounted to the outer surface of the housing is an alumina stiffener. The stiffener, ranging in thickness from about 0.38 to 0.5 inches, depending on the size of the housing, is sufficiently thick to minimize housing deflections. While the use of such stiffeners has reduced the vibratory bending of the housing, and hence the vibrations induced in the SAW device substrate, in some applications there is not enough space for the now larger housing-stiffener combination.